1. Field of the Invention
The present invention relates to a process for producing an Fe-based member having a high Young""s modulus and an Fe-based member having a high Young""s modulus and a high toughness.
2. Description of the Related Art
There is a conventionally known method for enhancing the Young""s modulus of an Fe-based member, which is to compound a dispersing material such as a reinforcing fiber, reinforcing granules and the like having a high Young""s modulus to a matrix for the Fe-based member.
However, the known method suffers from problems that the dispersing material is coagulated in the matrix, and that when the surface properties are poor, the toughness of the Fe-based member is largely injured.
Accordingly, it is an object of the present invention to provide a producing process of the above-described type, wherein a particular metallographic structure can be produced by subjecting an Fe-based material having a particular composition to a particular treatment, thereby mass-producing an Fe-based member having a high Young""s modulus, a high toughness or a toughness required for practical use.
To achieve the above object, according to the present invention, there is provided a process for producing an Fe-based member having a high Young""s modules, comprising a first step of subjecting an Fe-based material comprising
0.6% by weightxe2x89xa6carbon (C)xe2x89xa61.9% by weight
silicon (Si) less than 2.2% by weight
0.9% by weightxe2x89xa6manganese (Mn)xe2x89xa61.7% by weight
0.5% by weightxe2x89xa6nickel (Ni)xe2x89xa61.5% by weight and
the balance of iron (Fe) including inevitable impurities, to a thermal treatment at a heating temperature T1 set in a range of TS less than T1 less than TL wherein TS represents a solidus temperature for the Fe-based material and TL represents a liquidus temperature, and under a cooling condition set at a quenching level, and a second step of subjecting the resulting Fe-based material to a thermal treatment at a heating temperature T2 set in a range of Te1 less than T2 less than Te2 wherein Te1 represents a eutectic transformation-starting temperature, and Te2 represents a eutectic transformation-finishing temperature, and for a heating time t set in a range of 60 minxe2x89xa6txe2x89xa6180 min.
If the Fe-based material having the above-described composition is subjected to the thermal treatment at the first step, the solidified structure is transformed into a primary thermally treated structure. The primary thermally treated structure is comprised of a matrix comprising martensite, a large number of massive residual xcex3 phases, a large number of intermetallic compound phases and the like. If the conditions are changed at the first step, the primary thermally treated structure cannot be produced. In the quenching, the cooling rate CR is set higher than a usual oil-cooling level or a forcibly air-cooling level, and thus, at CRxe2x89xa7250xc2x0 C./min. For this quenching, for example, an oil-cooling, a water-cooling or the like may be used.
If the Fe-based material having the primary thermally treated structure is then subjected to the thermally treatment at second step, the primary thermally treated structure is transformed into a secondary thermally treated structure. The secondary thermally treated structure is comprised of a matrix, for example, comprising an xcex1 phase, a large number of fine carbide granules, a large number of massive precipitated xcex3 phases and the like. Fine short fiber-shaped carbide phases may be included in the secondary thermally treated structure in some cases.
In the secondary thermally treated structure, the fine carbide granules contribute to an enhancement in Young""s modulus of the Fe-based member, and the precipitated xcex3 phases contribute to an enhancement in toughness of the Fe-based member.
If the heating temperature T2 is lower than Te1 or the heating time t is shorter than 60 minutes at the second step, the fine division and dispersion of the carbide cannot be achieved sufficiently. On the other hand, if the heating temperature T2 is higher than Te2 or the heating time t is longer than 180 minutes at the second step, the graphitization is advanced excessively, and the coagulation of the carbide is produced.
Carbon (C) in the composition of the Fe-based material produces the fine carbide granules which contribute to an enhancement in Young""s modulus. To increase the amount of fine carbide granules produced, it is necessary to add a larger amount of carbon(C), and hence, the lower limit of the C content is set at 0.6% by weight. On the other hand, if C greater than 1.9% by weight, not only the carbide content but also the graphite content are increased and further, a eutectic graphite phase is precipitated. For this reason, the Fe-based member is embrittled.
Silicon (Si) serves to promote the deoxidation and the graphitization and is dissolved as a solid solution into the xcex1 phase to reinforce the xcex1 phase. In addition, silicon (Si) has an effect of increasing the difference xcex94T between the eutectic transformation starting temperature Te1 and the eutectic transformation finishing temperature Te2, namely, widening the range of the heating temperature T2 at the second step. Therefore, it is desired to increase the silicon content, but if the silicon content is increased, the graphite content is increased because of the larger C content. Thus, the Si content is set at Si less than 2.2% by weight, preferably, at Sixe2x89xa61.0% by weight.
Manganese (Mn) has an effect of promoting the deoxidation and the production of carbide and increasing the above-described temperature difference xcex94T. Nickel (Ni) which is another alloy element has an effect of inhibiting the production of carbide. Therefore, the lower limit value of the Mn content is set at 0.9% by weight in order to overcome such effect of nickel (Ni) to promote production of carbide. On the other hand, if Mn greater than 1.7% by weight, the Fe-based member is embrittled.
Nickel (Ni) is a xcex3-phase producing element, and has an effect of permitting a small amount of precipitated xcex3 phases to exist at ambient temperature to confine impurities in the precipitated xcex3 phases, thereby enhancing the toughness of the Fe-based member. To provide such an effect, it is desirable to set the Ni content at about 1% by weight. In addition, nickel (Ni) exhibits a significant effect of increasing the temperature difference xcex94T. However, when the nickel(Ni) content is set at Ni less than 0.5% by weight, the above effects cannot be obtained. On the other hand, even if the nickel content is set at Ni greater than 1.5% by weight, the increment of the temperature difference xcex94T is not varied.
In addition, according to the present invention, there is provided a process for producing an Fe-based member, wherein the heating temperature relative to the liquidus temperature TL is set at T1 greater than TL and a quenching similar to that described above is carried out at a first step, and then, a second step similar to that described above is carried out, as well as a process for producing an Fe-based member, wherein the heating temperature relative to an Acm temperature and the solidus temperature TS is set in a range of TAxe2x89xa6T1xe2x89xa6TS at a first step, and the second step similar to that described above is carried out.
Even with these processes, a thermally treated structure similar to the above-described secondary thermally treated structure can be produced.
Further, according to the present invention, there is provided a process for producing an Fe-based member having a high Young""s modulus and a high toughness, comprising a first step of subjecting an Fe-based material comprising
0.6% by weightxe2x89xa6carbon (C)xe2x89xa61.9% by weight
silicon (Si) less than 2.2% by weight
0.9% by weightxe2x89xa6manganese (Mn)xe2x89xa61.7% by weight
0.5% by weightxe2x89xa6nickel (Ni)xe2x89xa61.5% by weight
Ni (% by weight)/Mn (% by weight)xe2x89xa61.12 and
the balance of iron (Fe) including inevitable impurities, to a thermal treatment at a heating temperature T1 set at T1xe2x89xa7TA wherein TA represents an Acm temperature for the Fe-based material, and under a cooling condition set at a quenching level, and a second step of subjecting the resulting Fe-based material to a thermal treatment at a heating temperature T2 set in a range of TS1xe2x89xa6T2xe2x89xa6TS2 wherein TS1 represents a temperature when the amount of carbon solid solution in a matrix of the Fe-based material is 0.16% by weight, and TS2 represents a temperature when the carbon solid solution is 0.40% by weight.
If the Fe-based material having the above-described composition is subjected to the thermal treatment at the first step, the solidified structure is transformed into a primary thermally treated structure. The primary thermally treated structure is comprised of a matrix comprising, for example, martensite, a large number of massive residual xcex3 phases and the like. If the conditions are changed at the first step, a primary thermally treated structure as described above cannot be produced. In the quenching, the cooling rate CR is set higher than a usual oil-cooling level or a forcibly air-cooling level, and thus, at CRxe2x89xa7250xc2x0 C./min. For this quenching, for example, oil-cooling, water-cooling or the like may be used.
If the Fe-based material having the primary thermally treated structure is then subjected to the thermally treatment at second step, the primary thermally treated structure is transformed into a secondary thermally treated structure. At the second step, the amount of carbon dissolved as a solid solution into the matrix is suppressed into a range of 0.16% by weightxe2x89xa6SCxe2x89xa60.40% by weight, in accordance with this, the precipitation of the fine granular carbide is promoted. Therefore, the secondary thermally treated structure is comprised of a matrix comprising, for example, an xcex1 phase, a large number of fine carbide granules, a large number of graphite grains, a large number of massive precipitated xcex3 phases and the like. The heating time t at the second step is suitable to be in a range of 30 minxe2x89xa6txe2x89xa6180 min. Fine short fiber-shaped carbide phases may be included in the secondary thermally treated structure in some cases.
In the secondary thermally treated structure, the fine carbide granules contribute to an enhancement in Young""s modulus of the Fe-based member, and the precipitated xcex3 phases contribute to an enhancement in toughness of the Fe-based member.
If the heating temperature T2 at the second step is lower than TS1, the amount CS of carbon solid solution in the matrix is smaller, and the amount of the fine carbide granules is also smaller. On the other hand, if the heating temperature T2 is higher than TS2, the carbon solid solution is increased, but the amount of fine carbide granules precipitated is decreased. The heating time t shorter than 30 minutes corresponds to a case where T2 less than TS1, and t greater than 180 minutes corresponds to a case where T2 greater than TS2.
In the composition of the Fe-based material, carbon (C) produces the fine carbide granules contributing to an enhancement in Young""s modulus. To increase the amount of fine carbide granules produced, it is necessary to add large amount of carbon (C), and hence, the lower limit of the C content is set at 0.6% by weight. On the other hand, if C greater than 1.9% by weight, not only the carbide content but also the graphite content are increased and further, a eutectic carbide and a eutectic graphite are precipitated. For this reason, the Fe-based member is embrittled. To enhance the Young""s modulus and the toughness of the Fe-based member, the C content is preferably smaller than 1.0% by weight.
Silicon (Si) serves to promote the deoxidation and the graphitization and is dissolved as a solid solution into the xcex1 phase to reinforce the xcex1 phase. If the silicon content is increased, the graphite content is increased because of the larger C content. Thus, the Si content is set at Si less than 2.2% by weight, preferably, at Sixe2x89xa61.0% by weight.
Manganese (Mn) has an effect of promoting the deoxidation and the production of fine carbide granules and widening the area where the xcex1-, xcex3- and graphite-phases coexist. However, the Mn content is smaller than 0.9% by weight, the amount of carbide produced is decreased. On the other hand, if Mn greater than 1.7% by weight, the Fe-based member is embrittled.
Nickel (Ni) is a xcex3-phase producing element, and has an effect of permitting a small amount of precipitated xcex3 phases to exist at ambient temperature to confine impurities in the precipitated xcex3 phases, thereby enhancing the toughness of the Fe-based member. To provide such an effect, it is desirable to set the Ni content at about 1% by weight. In addition, nickel (Ni) exhibits a significant effect for increasing a temperature difference xcex94T between the temperatures TS1 and TS2. However, if the nickel content is smaller than 0.5% by weight, both of such effects cannot be obtained. On the other hand, even if the Ni content is set at Ni greater than 1.5% by weight, the increment of the temperature difference xcex94T is not varied.
In this case, if the ratio of the Ni content to the Mn content is Ni (% by weight)/Mn (% by weight) greater than 1.12, the content of graphite in the Fe-based member is increased, resulting in a reduced Young""s modulus.
If required, aluminum (Al) and nitrogen (N) may be added to the Fe-based material in addition to the above-described alloy elements. Aluminum (Al) has an effect of promoting the deoxidation and widening the area where the xcex1-, xcex3- and graphite-phases coexist, as does manganese (Mn), and is an a phase and graphite producing element. The usual upper limit value of the Al content is 1.2% by weight. A small amount of nitrogen (N) added exhibits an effect widening the area where the xcex1-, xcex3- and graphite-phases coexist. However, if nitrogen (N) is not completely dissolved as a solid solution into the matrix, it causes voids to be produced, resulting in degraded mechanical properties of the member, and it becomes a nucleus for graphite, thereby bringing about an increase in graphite content. Therefore, the upper limit value of the N content is set at 0.45% by weight.
It is another object of the present invention to provide the producing process of the above-described type, wherein a particular metallographic structure can be produced by subjecting an Fe-based material having a particular composition to a particular thermal treatment, thereby mass-producing an Fe-based member which has both of a high Young""s modulus and a high toughness; has a good cold workability and moreover, has mechanical properties which are not degraded.
To achieve the above object, according to the present invention, there is provided a process for producing an Fe-based member having a high Young""s modulus and a high toughness, comprising a first step of preparing an Fe-based material comprising
0.6% by weightxe2x89xa6carbon (C)xe2x89xa61.0% by weight
silicon (Si) less than 2.2% by weight
0.9% by weightxe2x89xa6manganese (Mn)xe2x89xa61.7% by weight
0.5% by weightxe2x89xa6nickel (Ni)xe2x89xa61.5% by weight
Ni (% by weight)/Mn (% by weight)xe2x89xa61.12
0.3% by weightxe2x89xa6AExe2x89xa61.5% by weight and
the balance of iron (Fe) including inevitable impurities, wherein AE is at least one alloy element selected from the group consisting of Ti, V, Nb, W and Mo,
and subjecting the Fe-based material to a thermal treatment at a heating temperature T1 set at T1xe2x89xa7TA3 wherein TA3 represents the A3 temperature of the Fe-based material and under a cooling condition set at a quenching level, and a second step of subjecting the resulting Fe-based material to a thermal treatment at a heating temperature T2 set in a range of TS1xe2x89xa6T2xe2x89xa6TS2 wherein TS1 represents a temperature when the amount of carbon solid solution in a matrix of the Fe-based material is 0.16 and by weight, and TS2 represents a temperature when the amount of carbon solid solution is 0.40% by weight.
If the Fe-based material having the above-described composition is subjected to the thermal treatment at the first step, the solidified structure is transformed into a primary thermally treated structure. The primary thermally treated structure is comprised of a matrix comprising, for example, martensite, a large number of massive residual xcex3 phases and the like. If the conditions are changed at the first step, a primary thermally treated structure as described above cannot be produced. In the quenching, the cooling rate CR is set higher than a usual oil-cooling level or a forcibly air-cooling level, and thus, at CRxe2x89xa7250xc2x0 C./min. For this quenching, for example, oil-cooling, water-cooling or the like may be used.
If the Fe-based material having the primary thermally treated structure is then subjected to the thermally treatment at second step, the primary thermally treated structure is transformed into a secondary thermally treated structure. At the second step, the amount SC of carbon dissolved as a solid solution into the matrix is suppressed into a range of 0.16% by weightxe2x89xa6SCxe2x89xa60.40% by weight, and in accordance with this, the precipitation of fine granular carbide is promoted, whereby the matrix is transformed into a hypo-eutectic structure in cooperation with an effect of the alloy element AE. Therefore, the secondary thermally treated structure is comprised of a large number of fine carbide granules, a large number of graphite grains, a large number of massive precipitated xcex3 phases and the like which are dispersed in a matrix of the hypo-eutectic structure. The heating time t at the second step is suitable to be in a range of 30 minxe2x89xa6txe2x89xa6180 min. Fine short fiber-shaped carbide phases may be included in the secondary thermally treated structure in some cases.
In the secondary thermally treated structure, the fine carbide granules contribute to an enhancement in Young""s modulus of the Fe-based member, and the precipitated xcex3 phases contribute to an enhancement in toughness of the Fe-based member. If the welding is carried out when the matrix is of a hyper-eutectic structure, a net-shaped carbide phase is produced, resulting in degraded mechanical properties. However, such disadvantage is avoided by transforming the matrix into the hypo-eutectic structure, as described above.
If the heating temperature T2 is lower than TS1 at the second step, the amount of fine carbide granules precipitated is smaller. On the other hand, if the heating temperature T2 is higher than TS2, the amount CS of carbon solid solution is increased, but the amount of fine carbide granules precipitated is decreased. The heating time t shorter than 30 minutes corresponds to a case where T2 less than TS1, and T greater than 180 minutes corresponds to a case where T2 greater than TS2.
Carbon (C) in the composition of the Fe-based material produces the fine carbide granules which contribute to an enhancement in Young""s modulus. To increase the amount of fine carbide granules produced, it is necessary to add large amount of carbon(C), and hence, the lower limit of the C content is set at 0.6% by weight. On the other hand, if C greater than 1.0% by weight, the carbide content is too large and for this reason, the Fe-based member is embrittled.
Silicon (Si) serves to promote the deoxidation and the graphitization and is dissolved as a solid solution into the xcex1 phase to reinforce the xcex1 phase. If the silicon content is increased, the graphite content is increased. Therefore, the Si content is set at Si less than 2.2% by weight, preferably, at Sixe2x89xa61.0% by weight.
Manganese (Mn) has an effect of promoting the deoxidation and the production of carbide and widening the area where the xcex1-, xcex3- and graphite phases coexist. However, if the Mn content is less than 0.9% by weight, the amount of carbide produced is decreased. On the other hand, if Mn greater than 1.7% by weight, the Fe-based member is embrittled.
Nickel (Ni) is a xcex3-phase producing element, and has an effect of permitting a small amount of precipitated xcex3 phases to exist at ambient temperature to confine impurities in the precipitated xcex3 phases, thereby enhancing the toughness of the Fe-based member. To provide such an effect, it is desirable to set the Ni content at about 1% by weight. In addition, nickel (Ni) exhibits a significant effect for increasing the temperature difference xcex94T between the temperatures TS1 and TS2. Further, nickel (Ni) has an effect for enhancing the elongation of the Fe-based member at ambient temperature, and enhancing the flexure characteristic to improve the cold workability. However, if the nickel content is set smaller than 0.5% by weight, the above-described effects cannot be obtained. On the other hand, even if the Ni content is set at Ni greater than 1.5% by weight, the increment of the temperature difference xcex94T is not varied.
In this case, if the ratio of the Ni content to the Mn content is Ni (% by weight)/Mn (% by weight) greater than 1.12, the amount of graphite in the Fe-based member is increased, resulting in a reduced Young""s modulus.
Ti, V, Nb, W and Mo which are alloy elements AE have an effect of producing carbide at an early stage and reducing the concentration of C in the matrix to transform the matrix into the hypo-eutectic structure, because they are more active than Fe and Mn. Thus, it is possible to prevent the degradation of the mechanical properties of the Fe-based member due to the welding, and to enhance the cold workability of the Fe-based member. Particularly, there is an advantage that Ti also has a deoxidizing effect, and the titanium carbide has a specific rigidity. Further, if two or more of the alloy elements AE are added in combination, a carbide finely-dividing effect is exhibited. In this case, Ti and Nb produce carbides earlier than the finish of the solidification of the xcex3 phase and hence, such carbides act as nuclei for the xcex3 phase. Therefore, there is not raised such a disadvantage that the carbide of Ti and Nb exist in the crystal boundary to retard the toughness of the Fe-based member. On the other hand, carbides of V, W and Mo are dissolved as solid solutions into the xcex3 phase and precipitated in the granular forms and hence, it is possible to suppress the reduction in toughness of the Fe-based member to the minimum.
However, if the content of the alloy element AE is less than 0.3% by weight, the matrix is transformed into a hyper-eutectic structure and hence, this content is not preferred. On the other hand, if AE greater than 1.5% by weight, the amount of the carbide existing in the crystal boundary between the xcex3 phases is more than 2% in terms of the volume fraction Vf and for this reason, the toughness of the Fe-based member is retarded. The upper limit value of the Ti content is 1.2% by weight, and the upper limit value of the V content is 1.27% by weight.
In addition to the above-described alloy elements, if required, aluminum (Al) and nitrogen (N) may be added to the Fe-base material. Aluminum (Al) has an effect of promoting the deoxidation and widening the area where the xcex1-, xcex3- and graphite-phases coexist, as does manganese. In addition, aluminum (Al) is an xcex1 phase and graphite producing element. The usual upper limit value of the Al content is 1.2% by weight. A small amount of nitrogen (N) added exhibits an effect of widening the area where the xcex1-, xcex3- and graphite-phases coexist. However, if nitrogen (N) is completely not dissolved as a solid solution, it produces voids to degrade the mechanical properties of the member, and it becomes a nucleus to bring out an increase in content of graphite. Therefore, the upper limit value of the N content is set at 0.45% by weight.
The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings.